1. Technical Field
This invention relates to methods for preparing soft magnetic thin films based on cobalt and iron, and soft magnetic thin films obtained therefrom.
2. Background Art
Soft magnetic thin films are currently on widespread use in electronic applications such as electronic parts including thin-film magnetic heads, thin-film inductors and thin-film transformers. In particular, in order for a thin-film magnetic head to perform high density magnetic recording, it is necessary to shrink recorded bits, which in turn, requires the thin-film magnetic head to produce a high strength magnetic field for writing. Then the soft magnetic thin film used in the thin-film magnetic head must be formed of a magnetically soft material having a high saturation flux density (Bs). With respect to thin-film inductors and thin-film transformers which are required to reduce their size and the thickness of film, a demand for a magnetically soft material having a high saturation flux density exists like the thin-film magnetic heads.
Magnetic thin films having a high saturation flux density are known in the art. For example, Japanese Patent No. 2,821,456 discloses a method for preparing a soft magnetic thin film of CoNiFe having a saturation flux density of 1.7 to 2.1 tesla (T) by electroplating. JP-A 2000-322707 discloses a method for preparing a soft magnetic film of CoFeNi having a saturation flux density of 2 to 2.3 T by electroplating.
Recently, for the purpose of increasing magnetic recording density or the like, there is a growing interest in the use of CoFe-based alloys having a high saturation flux density as compared with NiFe-based alloys and CoNiFe-based alloys used in the art as soft magnetic thin films.
The saturation flux density of CoFe alloys is described in R. M. Bozorth, “Ferromagnetism,” D. Van Nostrand Co. Inc., N.Y., 1951. Theoretically, a saturation flux density of higher than about 2.2 T is available when the alloy composition is in the approximate range: 5 at %≦Co≦70 at % and 30 at %≦Fe≦95 at %. The saturation flux density reaches a maximum of about 2.4 T when the alloy consists of about 35 at % of cobalt and about 65 at % of iron. IEEE. Trans. Magn., 1987, vol. 23, p. 2981 describes that an electroplated CoFe alloy film formed of about 90 at % Co and about 10 at % Fe has a saturation flux density of about 1.9 T.
Also, IEEE. Trans. Magn., 2000, vol. 36, p. 3479 describes a CoFe alloy film composed of about 35 at % Co and about 65 at % Fe. Although this film has the composition alleged as exhibiting a highest saturation flux density, the saturation flux density is only about 2.0 T. That is, the saturation flux density is not so high as expected. This is probably because divalent Fe ions in the plating bath are oxidized.
It is then desirable to form a CoFe alloy film while controlling the oxidation of divalent Fe ions. For example, JP-A 6-96949 discloses a method for preparing a CoFe alloy film by electroplating in a plating solution while adding a reducing agent such as ascorbic acid, hypophosphorous acid, dimethylaminoboran, thiourea, or salts or derivatives thereof to the plating solution for preventing divalent Fe ions from being oxidized or for reducing trivalent Fe ions (formed as a result of oxidation) to a divalent state. The CoFe alloy film obtained by this method, however, has a less satisfactory saturation flux density.
The inventors proposed in Japanese Patent Application No. 2002-153252 to form a CoFe alloy film while adding a boron-based reducing agent to a plating solution for preventing divalent Fe ions from being oxidized. With this method, however, non-metallic components such as boron originating from the reducing agent are incorporated in the CoFe alloy film in addition to Co and Fe, detracting from its saturation flux density. A CoFe alloy film having an inherent saturation flux density is not available as well.